1. Field of the Invention
The present invention relates to a visual image reproducing device for use such as in a television receiver, a videotape recorder or a video projector and, more particularly, to an outline enhancing device which can be used to enhance the outline of a video image for a sharper image reproduction.
2. Description of the Prior Art
As demands for the quality of image reproduced by television has increased in recent years, the outline enhancing devices have become increasingly important as a means for sharpening the image reproduced on the CRT screen by enhancing the rising and falling slopes of video signal.
One type of such device is a described below with reference to the accompanying figures.
FIGS. 3 and 4 are, respectively, a block diagram and a graph showing the input/output relationship in an example of a conventional outline enhancing device. As shown in Fig. 3, the outline enhancing device OCp includes a differentiating circuit 10 which differentiates a video signal S1 for producing a first differentiated video Signal S2 '; a delay circuit 20 which delays the first differentiated video signal S2' for producing a first delayed differentiated signal S3'; a positive half-wave rectifying circuit 30 which rectifies the first differentiated video signal S2' for producing a positive first differentiated signal S4' by passing the positive element thereof only; a negative half-wave rectifying circuit 40 which rectifies the first differentiated video signal S2' for producing a negative first differentiated signal S5' by passing only the negative element thereof; a positive half-wave rectifying circuit 50 which rectifies the first delayed differentiated signal S3' for producing a positive first delayed differentiated signal S6' by passing only the positive element thereof; a negative half-wave rectifying circuit 60 which rectifies the first delayed differentiated signal S3' for producing a negative first delayed differentiated signal S7' by passing only the negative elements thereof; a multiplier 70 which produces a multiplied positive first differentiated signal S8' from the positive first differentiated signal S4' and the positive first delayed differentiated signal S6'; a multiplier 80 which produces a multiplied negative first differentiated signal S9' from the negative first differentiated signal S5' and negative first delayed differentiated signal S7'; a subtracter 90 which produces a multiplied first differentiated signal S10' by subtracting the multiplied negative first differentiated signal S9' from the multiplied positive first differentiated signal S8'; a differentiated circuit 100 which differentiates multiplied first differentiated signal S10' for producing a second differentiated signal S11'; a delay circuit 110 which delays the video signal S1 for producing a delayed video signal S12'; and a subtracter 120 which produces an outline enhanced signal S13' by subtracting the second differentiated signal S11' from the delayed video signal S12.'
A outline enhancing device thus constructed operates as described below, with references to FIG. 4.
First, the video signal S1 has a wave form having a first portion A, a second portion B and a third portion C such that it rises for a predetermined period Ta in the first portion A, stays at the risen level for a predetermined period Tb in the second portion, and goes down to zero level for a predetermined period Tc in the third portion C.
The differentiating circuit 10 differentiates video signal S1 and produces the first differentiated video signal S2'. The delay circuit 20 delays the first differentiated video signal S2' and thereby produces the first delayed differentiated signal S3'. Each of thus produced first differentiated video signal S2' and first delayed differentiated signal S3' is both divided into a positive and a negative signals such as signals S6', S4', S7, and S5', respectively, through the half-wave rectifying circuits 50, 60, 30, and 40. The positive signals S6' and S4' are multiplied to produce the multiplied positive first differentiated signal S8' by the multiplier 70. The negative signals S7' and S5' are also multiplied to produce the multiplied negative first differentiated signal S9'. Thus produced multiplied positive and negative signals S8' and S9' are converted to the multiplied first differentiated signal S10' which is in the form of a first differentiated signal. This first differentiated signal S10' is further differentiated by the differentiating circuit 10 to produce the second differentiated signal S11'. The second differentiated signal S11' is used to subtract from the delayed video signal S12' to produce the outline enhanced signal S13'. The second differentiated signal S11' has three portions Ap, Bp, and Cp having lengths of ap, bp, and cp, respectively, as shown in FIG. 4. Each of the first and third portion Ap and Bp includes both of positive and negative peaks PS' and OS'. Since the delayed video signal S12' is the same as the video signal S1 being delayed by the delay circuit 110 so as to synchronize with the second differentiated signal S11'. Since these portions ap and cp having positive and negative peaks PS' and OS' is used to enhance the gradients of the rising and falling slopes of the delayed video signal S12', thus obtained outline enhanced signal S13' has the steeper slopes when compared with the original video signal S1, resulting the sharpened outline of video image.
However, the outline enhancing device OCp does not include any means to secure that ap and cp equals to Ta and Tc of the video signal S1, respectively. Therefore, the lengths ap and cp may be any value greater or smaller than Ta and Tc. In such case, when these portions of ap, bp and cp invades neighboring area of the corresponding portions A, B, and B of the delayed video signal S12', respectively. The peaks PS' and OS' of the second differentiated signal S1140 cause preshoots PS and overshoots OS near the rising and falling slopes of the outline enhanced signal S13' as shown. Since these preshoots PS and overshoots OS are also reproduced around the outline area of the original image on the CRT, the quality of reproduced image of outline enhanced signal S13' is degraded.